17-08-2012, 02:31 PM
(This post was last modified: 17-08-2012, 03:15 PM by Robert Darwent.)
(17-08-2012, 09:27 AM)Joe Wrote: That soon popped too so I put a diode in series with the next one. That was a bit dim so I replaced it with a 4.5V one. This looks like it's at full brightness but according to both analogue and digital meters there's only just over 2V across it.
What the devil is going on?
Hi Joe,
In addition to the information provided by Lawrence, a quick method you can do in your head to arrive at a close value for the 'actual' voltage following a diode dropper; is to multiply whatever your meter measures by 1.57 (actually 1.11 x square root of 2), but 1.5 is more than accurate enough.
So with the 4.5V bulb and diode you are using, there will be roughly 2 x 1.5 = 3V across it. Since you say it is giving a good brightness and is being under-run by about a third voltage-wise, it should last a good while longer than the earlier ones you used.
Regards
